Bug 1277814 - Avoid division-by-zero when the cumulative resolution is zero. r=tnikkel

The division-by-zero was introducing NaNs into the displayport calculations,
resulting in bad displayports.

MozReview-Commit-ID: 5dbqIEOFADS

--HG--
extra : source : 569276f2edd39600de534183e12b7f8762b917c8

gfx/layers/FrameMetrics.h

@@ -179,11 +179,17 @@ public:
 
   CSSSize CalculateCompositedSizeInCssPixels() const
   {
+    if (GetZoom() == CSSToParentLayerScale2D(0, 0)) {
+      return CSSSize();  // avoid division by zero
+    }
     return mCompositionBounds.Size() / GetZoom();
   }
 
   CSSRect CalculateCompositedRectInCssPixels() const
   {
+    if (GetZoom() == CSSToParentLayerScale2D(0, 0)) {
+      return CSSRect();  // avoid division by zero
+    }
     return mCompositionBounds / GetZoom();
   }
 

gfx/layers/apz/src/AsyncPanZoomController.cpp

@@ -2697,7 +2697,10 @@ const ScreenMargin AsyncPanZoomController::CalculatePendingDisplayPort(
   }
 
   CSSSize compositionSize = aFrameMetrics.CalculateBoundedCompositedSizeInCssPixels();
-  CSSPoint velocity = aVelocity / aFrameMetrics.GetZoom();
+  CSSPoint velocity;
+  if (aFrameMetrics.GetZoom() != CSSToParentLayerScale2D(0, 0)) {
+    velocity = aVelocity / aFrameMetrics.GetZoom();  // avoid division by zero
+  }
   CSSRect scrollableRect = aFrameMetrics.GetExpandedScrollableRect();
 
   // Calculate the displayport size based on how fast we're moving along each axis.

layout/base/nsLayoutUtils.cpp

@@ -1000,6 +1000,19 @@ GetDisplayPortFromMarginsData(nsIContent* aContent,
   LayoutDeviceToScreenScale2D res(presContext->PresShell()->GetCumulativeResolution()
                                 * nsLayoutUtils::GetTransformToAncestorScale(frame));
 
+  // Calculate the expanded scrollable rect, which we'll be clamping the
+  // displayport to.
+  nsRect expandedScrollableRect =
+    nsLayoutUtils::CalculateExpandedScrollableRect(frame);
+
+  // GetTransformToAncestorScale() can return 0. In this case, just return the
+  // base rect (clamped to the expanded scrollable rect), as other calculations
+  // would run into divisions by zero.
+  if (res == LayoutDeviceToScreenScale2D(0, 0)) {
+    // Make sure the displayport remains within the scrollable rect.
+    return base.MoveInsideAndClamp(expandedScrollableRect - scrollPos);
+  }
+
   // First convert the base rect to screen pixels
   LayoutDeviceToScreenScale2D parentRes = res;
   if (isRoot) {
@@ -1113,8 +1126,6 @@ GetDisplayPortFromMarginsData(nsIContent* aContent,
   result = ApplyRectMultiplier(result, aMultiplier);
 
   // Make sure the displayport remains within the scrollable rect.
-  nsRect expandedScrollableRect =
-    nsLayoutUtils::CalculateExpandedScrollableRect(frame);
   result = result.MoveInsideAndClamp(expandedScrollableRect - scrollPos);
 
   return result;